I'm doing research into vacuum distillation and the one area that im finding it difficult to get information on is what pressure to put the biodiesel under and what temperature I want to bring it to for full, or close to it, recovery of methanol. If any one could get me information on that or links that would be great.

I have been doing vacuum distillation for three years now prior to waterless wash/filtering of soaps.

I started using the vac pump off of a refrigerator, but now have a belt driven vac pump I found on Craigslist.

The temp of my bio is about 110-130 when I start distillation - I add no heat. I have a gauge in the system, and usually start to see methanol drawing through the plumbers delight condenser when the gauge reads 20"Hg. I let it run up to 28 or 29". It takes about an hour to get 1 gallon of methanol out. My typical 55 gallon of bio starts with 9 gallons of methanol. I bubble out the last bit of methanol in an open container where I also let the soaps settle out and or use a centrifuge to remove them.

I don't know how accurate my gauge is, I have had the same gauge for three years now.

Sinbad, thanks for posting those figures. Interesting that for demething both gly and bio, some steep temperatures are required at low methanol concentrations even under hard vacuum. Like Fabricator experienced with his centrifuge experiments, getting that last little bit out is problematic.

jon h., I'm sure I've asked you in the past, but what make and model is your vac pump - dry or lubricated vane? Do you think bubbling removes that last little bit of methanol?

Actual commercial produciton experience tells me that those numbers are wrong. There are many variables (surface area of the liquid, mixing, spraying, etc.) that affect the results. In fact, every piece of equipment performs a little differently due to these variables.

The only way I could see those numbers as accurate is if there was no mixing under those conditions. In that case those results would make sense.

Those numbers could very well be right (I haven't checked them myself yet) but they are just the tip of the iceberg when dealing with distillation, vacuum or otherwise. Making it as simple as possible, reflux is going to be your biggest help. Note that a vertical metal or an angled metal pipe above your still will count as a reflux. To get a specific reflux ratio what you need to do is very tightly control your still at a temp and control the temp of that pipe. You can theoretically get any percent purity you want then (Each specific set up will have it's own max purity in real life).

If you are doing this in a batch process (with a bucket of glycerin or BD or something) then what you want to do is slowly heat the solution (raise it a couple of degrees every so often). And keep measuring the temperature of your metal 'reflux' pipe. Once you hit a certain temperature on the outlet stop collecting as the purity will be too low for re-use, and dispose of the rest as methanol-ish wash water.

I posted those numbers above years ago, under a different username that I lost the password for. I worked for a company at the time that had some chemical process simulation software. Those are the numbers for boiling point that it spit out, using Antione's equation. It was not experimental data. I think the point was that just because methanol boils at 148F doesn't mean that a solution with methanol will start to boil at 148F. The boiling point of a solution is a function of all components of the solution, kind of an average.

I've got some commercial experience now too that I didn't have then. I'd say that under vacuum, 250F will get out enough moisture and methanol to be in spec.

please pardon the non vacuum question..if I'm reading this correctlyif I stop collecting at 205F there is roughly 20% methanol left in solution?since I start seeing methanol coming out around 180F or so..then I'm only removing 20% of the methanol..are am I off the wall here? I'm asking because I know my demethed glycerin has a flash point higher than 140F and isn't considered hazmat material. also I thinking with the new viton seal in my pump I can run the temp up to 230-240F without problems. but this should net me only 10% more methanol..

for me this issue is the stick wicket..and why I don't use vacuum distillation..how to maintain purity under a vacuum? as the vacuum level changes the temp in the reflux column needs to change. this mean controlling 2 variables instead of 1.